1. Technical Field
Exemplary embodiments of this disclosure relate to a transfer device incorporated in an electrophotographic image forming apparatus such as a copier, a printer, a facsimile machine, or a multi-functional device having at least one of the foregoing capabilities, and an electrophotographic image forming apparatus incorporating the transfer device.
2. Description of the Related Art
Conventionally, among color electrophotographic image forming apparatuses, there have been widely used an apparatus including a transfer device that transfers, onto a recording medium, toner images carried on a plurality of photoconductors and image carriers such as intermediate transfer bodies, for the purpose of high image quality and high reliability.
In a configuration using an intermediate transfer member, the transfer device secondarily transfers a toner image carried on the intermediate transfer member onto a recording medium by applying a secondary transfer bias when the recording medium passes through between the intermediate transfer member and a secondary transfer roller, in a state of a predetermined nipping pressure being generated in a secondary transfer nipping portion formed by the intermediate transfer member and the secondary transfer roller. On the other hand, when a secondary transfer is not performed in a waiting time of the image forming apparatus and in an adjustment operation inside the image forming apparatus, the secondary transfer roller is moved to be separated from the intermediate transfer member in order to prevent deterioration of the transfer device having the intermediate transfer member and the secondary transfer roller. There has been conventionally known a transfer device having a moving unit which moves (contacts and separates) the secondary transfer roller to a state of the secondary transfer nipping portion being formed by abutting the secondary transfer roller against the intermediate transfer member and a state of the intermediate transfer member and the secondary transfer nipping portion being separated.
For example, an image forming apparatus includes a transfer device (secondary transfer device) having the following moving unit (contact-and-separation unit) that moves the secondary transfer roller so that an intermediate transfer belt as a belt-shaped intermediate transfer member and the secondary transfer roller are contacted and separated. The moving unit includes a biasing unit that biases the secondary transfer roller toward the intermediate transfer belt, and cams that move the secondary transfer roller so that the secondary transfer roller is separated from the intermediate transfer belt against the bias force of the biasing unit and that the secondary transfer roller is brought into contact with the intermediate transfer belt by the bias force of the biasing unit. The cams are plate cams provided in a rotary shaft that rotatably supports a secondary-transfer opposing roller that opposes the secondary transfer roller via the intermediate transfer belt. A distance between an outer circumferential surface of each cam at a position where the cam opposes or is in contact with a follower (roller) provided on the rotary shaft of the secondary transfer roller and a rotation center of the cam (hereinafter, referred to as a “cam radius”) is changed by rotation so that a position of the follower in contact with the cam is moved. By moving the follower in this way, the secondary transfer roller is moved by changing an inter-axis distance between the secondary transfer roller and the secondary-transfer opposing roller.
On an outer circumferential surface of the cams, the following three kinds of regions are formed. A cam radius opposing the follower presses the follower against the bias force of the biasing unit, and is formed in a predetermined cam radius in which the intermediate transfer belt and the secondary transfer roller separate from each other, and the cam radius becomes in contact with the follower. This is a first region. The cam radius is formed in a predetermined cam radius in which the intermediate transfer belt and the secondary transfer roller are abutted against each other by the bias force of the biasing unit, and the cam radius is separated from the follower. This is a second region. The cam radius changes between the first region where the cam radius is in a state of being in contact with the follower and the second region where the cam radius is in a state of being separated from the follower. This is a third region. The moving unit changes the cam radius on the outer circumferential surface opposing the follower by rotating the cam configured as described above, moves the secondary transfer roller by changing the inter-axis distance between the secondary transfer roller and the secondary-transfer opposing roller, and contacts and separates the intermediate transfer belt and the secondary transfer roller.
Before a front end of the recording medium enters the secondary transfer nipping portion formed by the intermediate transfer belt and the secondary transfer roller, rotation of the cam for changing the outer circumferential surface opposing the follower from the first region to the second region is started at a certain angular velocity (velocity). After rotation starts, the angular velocity of the cam is accelerated to a target angular velocity by the time when the front end of the recording medium enters the secondary transfer nipping portion. Thereafter, the target angular velocity is maintained, and the front end of the recording medium enters the secondary transfer nipping portion in a state that the outer circumferential surface of the cam opposing the follower is in a state of being in the first region. By the rotation at the target angular velocity of the cam that changes the outer circumferential surface opposing the follower from the first region to the second region, the outer circumferential surface opposing the follower reaches the change region. Then, the inter-axis distance between the secondary transfer roller and the secondary-transfer opposing roller starts becoming short. When the inter-axis distance becomes shorter in this way, the intermediate transfer belt and the secondary transfer roller are abutted against each other via the recording medium, and a nipping pressure starts being generated in the secondary transfer nipping portion. Thereafter, the rotation of the cam at the target angular velocity is continued, and the secondary transfer roller moves toward the intermediate transfer belt to a position of generating a nipping pressure at which clear aggravation of a secondary transfer is not observed (hereinafter, referred to as “necessary nipping-pressure position”). Thereafter, a front end of the image-forming region of the recording medium enters the secondary transfer nipping portion.
In this way, the nipping pressure is generated in the secondary transfer nipping portion after the front end of the recording medium enters the secondary transfer nipping portion. Therefore, a rapid increase in the nipping pressure generated when a cardboard enters can be reduced more than that in a transfer device that makes the recording medium enter secondary transfer nipping portion in a state of the nipping pressure being generated. Before the front end of the image-forming region of the recording medium enters the secondary transfer nipping portion, the secondary transfer roller can be moved toward the intermediate transfer belt to the necessary nipping-pressure position. Therefore, by reducing a rapid increase in the nipping pressure generated when a cardboard enters more than an increase in a transfer device that makes the recording medium enter the secondary transfer nipping portion in a state of the nipping pressure being generated, generation of shock jitter can be suppressed and image quality reduction attributable to a transfer pressure shortage can be suppressed.
In recent years, there has been a growing demand by users for higher image quality in the image forming apparatus. Therefore, the transfer device that secondarily transfers a toner image on the intermediate transfer member after performing the primary transfer, to the recording medium, is also required to further enhance the suppression effect of suppressing image quality reduction such as shock jitter when making a cardboard and the like enter the secondary transfer nipping portion.